1. Field of the Invention
This invention relates to a method and apparatus to be lowered into a well, to determine the thickness of the pipe at localized areas in the well and also the electrical conductivity of the surrounding earth formation at selected depths in the well.
2. Description of Related Art
Progress in the field of electromagnetics has been with much travail and difficulty. Electromagnetic signals presently are being received from satellites at many millions of miles distance but this advancement was not accomplished in a single bound. Electromagnetic geophysical advancements have likewise come slowly.
In 1919, a patent was issued on induction geophysics. During the 1920's, Hans Lundburg and his group of Swedish scientists made a major geophysical find in Sweden with electromagnetic geophysics. They later failed to crack the oil patch geophysics in the late 1920's with inductive loops a quarter mile square.
Schlumberger worked on induction logging in the 1940's. According to Allaud and Martin, though Schlumberger was commercially logging in 1949, in was 1956 before they had what they considered a "solution". Thus the induction log took over forty five years of development from the patented idea of 1919 until Schlumberger's apparatus.
There are now three types of commercial magnetic inspection systems on the oil field market. All of them have been in the field for over 20 years. They are as follows:
1. Direct current magnetic devices: The pipe is polarized by a magnetic field being induced down the length of the pipe by an assembly of primary permanent magnets sometimes coupled to the pipe by steel wire brushes. The north and south poles of these magnets are spaced apart by about a foot along the length the pipe. Midway between the poles of the magnets there is an array of pickup coil assemblies which are distributed around the inside diameter of the pipe. This permanent magnet excited tool is chiefly used in pipeline pig surveys.
The same principle is used in oil wells where a direct current electromagnetic field replaces the permanent magnet and the brushes are eliminated. The signal power generated by the pickup coils in these tools is speed dependent, like the permanent magnet tools, and rises as the square of the speed that the tool moves through the pipe. The shape of the anomaly in the pipe as well as its depth within the pipe wall is also a factor. Thus, any interpretation of the nature of the casing flaws is extremely difficult. Furthermore, in the brush type tool, there are three breaks for each anomaly. First, an electrical pulse is induced when the first brush reaches the anomaly, a second electrical pulse is induced when the pickup structure reaches the anomaly and a third electrical pulse is induced when the next brush leaves the anomaly. Also, there is a signal generated in each of the adjoining pickups over about 5 inches of circumference for a 5/8 inch hole. In larger 30 inch OD pipe one can vaguely define the hole's diameter or shape, but in an oil well where the most common casing is 51/2 inch OD pipe this lack of definition makes the anomaly almost undiscernible.
2. High frequency eddy current devices: These devices induce eddy currents into the inner surface of the pipe with high frequency exploration coils. They can deliver excellent resolution but are only effected by shallow defects such as pits or scale on the inside wall of the pipe. In the oil well type tools, the eddy current detectors are run along with the permanent magnet or direct current magnetic field pickups in hopes that the difference between anomalies inside of the pipe and somewhere in the wall of the pipe can be discerned.
3. Low frequency phase shift method: Commercially the most successful type of casing inspection is the low frequency, i.e. 60 hertz, phase shift method. An AC transmitter coil is excited inside of the pipe. At a distance from the exciter coil, say a foot or two away, a receiver coil is used to pick up a portion of the current induced in the pipe, and then measuring the phase shift at the receiver coils. The phase shift measured is proportional to the average wall thickness of the pipe between the transmitter and the receiver coil. The weight of the pipe in this span may be 20 or more pounds. Because this measurement averages the wall thickness between the coils, it is very good at measuring general corrosion or average wear of the pipe. However, most leaks are caused by pitting in the pipe or extensive wear. A tiny hole in the casing, which may equal only an ounce or less of metal, cannot be detected. That is enough to ruin an oil well. The process is not sensitive enough to measure an ounce out of 20 to 40 pounds which represents a fraction of 1% of the total. The tool can scarcely detect a 10% loss of metal. As a matter of fact three inch diameter holes through the pipe have been totally missed in oil wells with this tool. Not a little of this difficulty is caused by variations in pipe roundness, by pipe diameter, by mechanical strains present in the pipe, just how the pipe is cooled when manufactured and variations in the mill producing the pipe.
Even though this low frequency device is the best commercial device in the field, the companies that run it only claim a determination of three levels of discernment: which means in essence "probably good", "perhaps bad" and "probably bad".